The purpose of the present study is to investigate CNS injury among injection drug abusers co-infected with HIV and Hepatitis C (HCV). The study will incorporate btomarkers of hepatic disease, state-of-the-art neuroimaging methods, and neuropsychological outcomes. Recent preliminary studies, including data from our lab, indicate that HIV-HCV co-infection is associated with greater neurocognitive compromise than either disease alone. Studies conducted to date have not fully determined the mechanisms underlying the potentially synergistic effect of co-infection. Further, the neurofunctional underpinnings of these effects have not been identified using functional or structural neuroimaging. Recent data from our laboratory identified strong relationships between markers of hepatic disease and cognitive outcome as well as relationships between neuroimaging abnormalities and cognitive function in co-infected individuals. The present study will extend these pilot data and examine CNS abnormalities in co-infected individuals using laboratory, neuroimaging and neurocognitive data. To accomplish these goals we will recruit two groups of active injection drug abusers including: 1) 30 HIV mono-infected patients, and 2) 30 HIV-HCV co-infected patients. All individuals will undergo laboratory testing including assessment of hepatic disease and HIV disease in the periphery, functional MRI, structural neuroimaging and neurocognitive assessment. This cross-sectional association study will be the first transdiciplinary approach to HIV-HCV co-infection, and will provide important insight into the putative mechanisms underlying increased cognitive impairment in this population. [unreadable] [unreadable] [unreadable]